1. Field of the Invention
The invention relates to a TAB (tape automated bonding) tape having a peeling-prevention structure which achieves increased peeling strength at the interface between, for example, an electrically conductive layer of the tape, such as a copper layer, and an electrically insulating, supporting layer of the tape, such as a polyimide layer. This structure serves to prevent peeling of the electrically conductive layer which would otherwise occur as a result of the application of external stresses such as the thermal stress applied during the chip mounting process and the bending stress applied during the lead frame forming process.
2. Description of the Related Art
At present, in conjunction with increased integration of semiconductor integrated circuit devices (hereinafter called semiconductor chips), conventional wire bonding methods for mounting semiconductor chips onto chip packages are being replaced by wireless connection methods such as the combination of TAB (tape automatic bonding) and the flip chip method. The latter connection methods are now increasingly preferred because they achieve both significant reductions in connection time and enhanced connection reliability.
In a semiconductor chip connecting process combining the TAB technique and the flip chip method, a chip having solder bumps is to be mounted onto a TAB tape which includes, for example, two layers. One of these layers is an electrically insulating support layer of, for example, polyimide, while the other layer is an electrically conductive layer of, for example, copper. This copper layer is deposited onto, and bonded to, the polyimide layer using conventional sputtering techniques or electroless plating, without an intermediate bonding layer. The copper layer is also patterned, using conventional photolithographic techniques, to form copper circuit lines, contact pads and test pads. During mounting of the chip, the solder bumps on the chip are brought into contact with copper circuitry, e.g., contact pads or circuit lines, of the TAB tape and heated and reflowed so as to achieve bonding. The bonded chip and TAB tape are washed, and a liquid resin sealant is applied to the chip, and dried. Electrical conductivity tests are then performed on the resin encapsulated chip via the test pads, which are subsequently cut off.
During the above-mentioned reflow process, the heating temperatures employed often exceed 300.degree. C. Because the copper layer and the polyimide layer have significantly different coefficients of thermal expansion, the temperatures employed during the reflow process result in substantial stresses at the interface between the copper layer and the polyimide layer, causing the copper layer to peel off from the polyimide layers. This tendency to peel off is particularly prevalent in TAB tapes where the area of contact between the polyimide layer and the copper layer is large. This tendency is even more prevalent at the tail end of the copper layer containing the test pad section, after the test pad section has peeled, because any peel control exerted on the adjacent portion of the copper layer is lost.
FIG. 7 depicts a conventional TAB tape having a copper layer which has peeled off from the supporting polyimide layer. Numeral 7 represents the test pad section used in the electrical conductivity test. It is formed at the end of copper layer 1. No adhesive layer is interposed between copper layer 1, which includes the test pad section 7, and polyimide layer 2, which supports copper layer 1. As shown, test pad section 7 has warped in a direction separating it from the polyimide layer 2 due, for example, to thermal stress produced at the interface between the two layers.
The test pad section thus peeled off produces problems, as follows:
(1) Contact failures can be caused in the electrical conductivity test process, leading to the entire product being discarded. PA0 (2) In the process of cutting-off the test pads, the cutting mold may be broken because a cut-off remnant of the test pad section, which peeled off from the polyimide layer, may get caught in the cutting mold. PA0 (3) By losing the support of the polyimide layer, the quality of the chip circuit product itself is deteriorated.
The peeling of copper layer 1 from the polyimide layer 2 caused by the difference in the thermal expansion coefficients is further aggravated when an external bending stress is applied to a portion of the copper layer 1, to form a lead frame, as depicted in FIG. 8.